A significant fraction of all courses of external-beam radiotherapy have unplanned interruptions, i.e. scheduled treatments are missed. Because of the kinetics of cellular damage repair and tumor repopulation, designing extra or augmented treatments to compensate for the missed fractions is non-trivial. Since some individualized compensation regimen must be chosen, it should be rationalized using state-of-the-art radiobiology. In Phase I we wrote prototype software (RIC, the Radiotherapy Interruption Compensator) which produces a range of practical options for the radiotherapist to compensate for unplanned interruptions during an extended radiotherapy regimen. It calculates what doses are needed to produce equal tumor control (or, alternately, equal late normal-tissue complications) when a proposed interruption-compensated regimen is substituted for the originally planned protocol. The program is designed as an "expert system", making available to any practicing radiotherapist the expertise of leading practitioners, in a personal-computer based, user-friendly package. Having demonstrated feasibility, we propose, in this Phase II application, to continue the research effort initiated in Phase I. The goal is to produce, at the end of the Phase II period, a marketable product which is a) scientifically state-of-the-art, b) designed and written with appropriate quality control, c) robust to use, d) credible to practicing clinicians. PROPOSED COMMERCIAL APPLICATION: Potential market consists of more than 2,500 Radiation Oncology facilities worldwide. Potentially about 1-4 copies of the software purchased by each facility, at about $900 per copy.